The use of evanescent wave phenomena as a detection means in optically-based assays is known in the art. The total reflective spectroscopy (TRS) techniques of Myron Block (U.S. Pat. No. 4,447,546 and U.S. Pat. No. 4,558,014) use an evanescent wave to both excite a fluorescently tagged analyte and to detect the resulting fluorescence. As disclosed therein, the sensors are comprised of optical telecommunication-type fibers which have a core surrounded by a cladding, at least where the fiber is held. Another portion of the fiber has a bare (or naked) core which is coated with an immunochemically reactive substance, i.e., an antigen or an antibody. This basic sensor configuration can be found in other disclosures such as WO 83/01112 to T. Carter et.al. and U.S. Ser. No. 652,714 to D. Keck et. al.
The presence of a cladding or means to insure energy isolation has both positive and negative effects. On the positive side, the cladding prevents mode stripping where the fiber is held. This is especially important when one is using a sensor for evanescent wave detection because in some cases, i.e., fluorescent measurements, less than one percent of the excitation energy will return as a signal. Thus, cladding has been required to insure that signal stripping does not occur. However, the negative consequences of this approach include manufacturing difficulties in selectively stripping or adding cladding to a fiber and the inherent limitation the cladding imposes on the critical angle or numerical aperture (NA) of the sensor.
The critical angle .theta..sub.c of a fiber optic or waveguide device, in general, and an evanescent wave sensor, in particular, is determined by the differences between the refractive indices of the launching medium, propagating medium, and the surrounding medium. It refers to the maximum angle, with respect to the longitudinal axis of the waveguide, at which light can enter the waveguide and still be retained and propagated by the waveguide. In practice, the art refers more often to the numerical aperture (NA) of a waveguide rather than the critical angle. Mathematically, the relationship is as follows: EQU NA=N.sub.0 sin.theta..sub.c =(N.sub.1.sup.2 -N.sub.2.sup.2).sup.1/2
Where
N.sub.0 =refractive index of the launching medium PA1 N.sub.1 =refractive index of the propagating medium PA1 N.sub.2 =refractive index of the surrounding medium. (See FIG. 1).